Cemented carbides are well known for their unique combination of hardness, strength and abrasion resistance and are accordingly extensively used in industry as cutting tools, drawing dies and wear parts. They are produced by powder metallurgy techniques from one or more refractory carbides of Groups IV, V, and VI of the periodic table, and are bonded or cemented together by liquid phase sintering with one or more of the iron group metals. However, it is important to appreciate that certain problems associated with one group of the periodic table do not appear in connection with the other groups of the periodic table. It is true that group IV of the metal carbides (titanium carbide, zirconium carbide, hafnium carbide) show great similarities of microstructure and properties within the group, but are vastly different from the group VI (tungsten carbides, molybdenum carbide, chromium carbide) with respect to crystal structure, physical properties and chemical behavior. For example, there is a tendency for chromium carbides to form further complex carbides in the tungsten carbide-nickel system. There is no similar tendency for chromium to do so with a titanium carbide, nickel-molybdenum system. This distinction between the cemented carbide groups is important since the predictability of solving certain problems within one group cannot necessarily be related to that of the other group.
This invention is concerned with improving the plastic deformation of the cutting edge associated with cemented titanium carbide tools. Plastic deformation is a common mode of failure of these tools, particularly when machining conditions, such as high speed and high feed, produce excessive temperatures at the cutting tip and result in plastic yielding. This is one of the common modes of failure of all carbide tools. This alone provides good reason to improve their deformation resistance, particularly the roughing grade which is most susceptible to this problem. Of even greater possible significance, however, is the observation that, in intermittent cutting, local plastic yielding at the cutting edge of a carbide tool can result in tensile stresses at that edge during the cooler, non-cutting part of the cycle, which stresses are large enough to initiate thermal cracks. This being true, inhibiting the cutting edge from deforming plastically is the key step in providing increased resistance to thermal cracking in operations such as milling where severe thermal cycling takes place.
In confirmation of the above, it has been observed that additions of chromium to TiC-Ni-Mo materials improve their deformation resistance as well as their thermal crack resistance. Chromium additions are known to remain essentially in the binding alloy phase of these materials. Increased "stiffness" of the binder phase due to solid solution strengthening by the chromiun is the mechanism from which this benefit is derived. It has been shown that aluminum also has a potent effect, similar to but more powerful than chromium, in decreasing nose push (see U.S. patent application Ser. No. 575,300, commonly assigned to the same assignee of this application). But such teachings of the prior art are primarily directed to improving the plastic deformation of the binding alloy. It has now become apparent that plastic deformation of the carbide phase may also take place at elevated temperatures encountered during metal cutting. It is to this latter aspect that this invention is directed, as well as an overall improvement in the plastic deformation of the entire composition utilizing materials that work in synergism with the ingredients added to the binding alloy.
An accepted criterion for measuring resistance to plastic deformation at elevated temperatures is the nose push test, referred to below. The nose push test procedure is as follows: a cutting tool is used to machine a cylindrically shaped work piece at a 0.06 inch depth of cut and at a feed of 0.011 inch per revolution for a 2 minute duration. Deformation on the nose of the tool, e.g. nose push, is then measured by running the stylus of a profilometer over the nose of the tool at an angle of 30.degree. to a line drawn normal to the tool flank. Nose push is, in fact, a deflection due to the plastic condition of the tip of the tool. The nose push values are reliably associated with plastic deformation at the elevated temperature reached by the nose of the tool. They increase directly as the cutting speed increases, due to increasing temperature.
Presently used commercial titanium carbide roughing grade compositions typically render an excessive nose push valve while cutting 1045 steel of 180 Brinell hardness at tool speeds of 1000 SFPM, resulting in an undesirable deformation of approximately 0.007 inches. This amount of deformation is not satisfactory and for most metal cutting operations would be considered a failure of cutting edge. Similarly, presently used commercial roughing grade titanium carbide compositions would render an undesirable nose push value in excess of 0.003 inches when cutting 4340 steel of about 300 Brinell hardness at tool speeds of 600 SFPM. Moreover, presently used commercial semi-roughing grades will provide an excessive nose push value over 0.003 inches when cutting 4340 steel of 300 Brinell hardness and at tool speed of 600 SFPM. For commercially used finishing grades, it has been found that a nose push value in excess of 0.001 inches when machining 4340 steel of 300 Brinell hardness at tool speeds of 600 SFPM is undesirable.